Transmitting digital data over a telephone network or line requires the use of modems. In the sending modem, a carrier signal is modulated by the data. In the receiving modem, the signal is demodulated to restore the original data.
Because the transmission channel introduces distortion, the modem receiver employs an equalizer for compensating the resulting inter-symbol interference. For every transmission, the input amplifier and the equalizer must be set to reflect the current transmission characteristics of the channel. Furthermore, carrier synchronization and symbol timing recovery have to be performed.
For setting the parameters in the modem receiver, prior to actual data transmission, a so-called training sequence is sent whose elements are known to both, transmitter and receiver. The channel characteristics can then be derived from the received training signal, allowing the receiver to carry out initial gain setting, equalizer training, symbol synchronization, and acquisition of carrier phase and frequency.
Use of training sequences was described e.g. in an article by K. H. Mueller and D. A. Spaulding entitled "Cyclic equalization--a new rapidly converging equalization technique for synchronous data communication", published in Bell System Technical Journal, Vol. 54, No. 2, February 1975, pp. 369-406, and also in U.S. Pat. No. 4,089,061 (A. T. Milewski) entitled "Method and apparatus for determining the initial values of a complex transversal equalizer". In the systems described in these prior art references, a periodic training signal is used, which repeats periodically a basic short sequence. The length of the period is selected to correspond to the length of the equalizer delay line.
Of key interest in modem receiver training is the time required for the training operation. In particular, in multidrop polling systems where the control modem receives many short messages from different tributary modems, the receiver has to adapt to the characteristics of a different telephone channel for each message. The time required for training the modem receiver thus has a strong influence on data throughput. Similarly, in half-duplex transmission, the startup time of the receiving modem contributes significantly to the turn-around delay.
Known training procedures and arrangements require the receipt of several periods of the training signal.